KorSA from the <i>Streptomyces</i> Integrative Element pSAM2 Is a Central Transcriptional Repressor: Target Genes and Binding Sites

<jats:title>ABSTRACT</jats:title> <jats:p> pSAM2, a 10.9-kb mobile integrative genetic element from <jats:italic>Streptomyces ambofaciens</jats:italic> , possesses, as do a majority of <jats:italic>Streptomyces</jats:italic> conjugative plasmids, a <jats:italic>kil-kor</jats:italic> system associated with its transfer. The <jats:italic>kor</jats:italic> function of pSAM2 was attributed to the <jats:italic>korSA</jats:italic> gene, but its direct role remained unclear. The present study was focused on the determination of the KorSA targets. It was shown that KorSA acts as a transcriptional repressor by binding to a conserved 17-nucleotide sequence found upstream of only two genes: its own gene, <jats:italic>korSA</jats:italic> , and <jats:italic>pra</jats:italic> , a gene positively controlling pSAM2 replication, integration, and excision. A unique feature of KorSA, compared to Kor proteins from other <jats:italic>Streptomyces</jats:italic> conjugative plasmids, is that it does not directly regulate pSAM2 transfer. KorSA does not bind to the pSAM2 genes coding for transfer and intramycelial spreading. Through the repression of <jats:italic>pra</jats:italic> , KorSA is able to negatively regulate pSAM2 functions activated by Pra and, consequently, to maintain pSAM2 integrated in the chromosome. </jats:p>